Xerath and Vel'Koz and mana usage.

Are these champions supposed to be mana users? All they do in lane is spam their abilities and yet somehow even with the smallest amount left in their "mana bar" they still have enough to do a full rotation and more.

I know both champions like to get Morellonomicon as their first item, but even before they buy the lost chapter, they still spam their abilities like they are manaless.

Add in comet + scorch and manaflow band and these 2 champions in particular don't need to leave lane until they themselves choose to. No need to back for mana or health. I include health because they can just clear a wave and then go back under their tower and repeat.

Furthermore, why are there still champions who practically go oom after one rotation of spells, while others get to spam theirs seemingly forever??